Apparatus for detecting leaks in vacuum systems



Sept. 22, 1953 J. BLEARS ET AL 2,652,716 I APPARATUS FOR DETECTING LEAKS IN VACUUM SYSTEMS Filed Nov. 18 1949 Inventors JacK Bleav'a John H. Lech TheirAttorhey Patented Sept. 22, 1953 APPARATUSIFOR.DETECTINGLEAKS IN;

VACUUMTSYSTEMSL Jack Blea'rs', -Davenham} and John Henry'- Leek- Stretford;..Engiand,;assignors to General Electrio Gompanmza corporation iofiNew :YOrk;

Application November 18,1949, .Sei'ialNoJ 128,182" If.

In GreatiBritain November 29;:1948i .L'

This .inventionrelates to apparatus for detect- Withsaidevaleuated space along a path'in whichiufs ing leaks in vacuum systems; and in particular is disposeda substance -adapted. to? absorb by. to the detection of such leaks'by the application, chemical reaction the gas or-vapour -intended for s y inzthe form ofra 'spray-,-=ofa'preseleeted-='-- use as the probe medium. The difierenoe .of 1 3., gas ear vapour; generally referred to as. the pressure in the two gauges 'due to directaccess of probe" me'dium,'so that'it passes into the vacuprobe medium to the one gauge and -chemica1.. um systemalong'any leakagepath whichmay be absorption- 0f probemedium in passage i-toxthe avai1ab1e;-and :bythe use in association with'the other 'gauge' afiects the electricalxbalance of avacuum system of detecting means se1ective1y----- bridge circuit' to afiord;ani indicationioi leakage... responsiveto the probe medium though insensiinto the vacuum-systeme1. tive to air or otheria'mbient fluid which may nor- With -the improved arrangement jt:.is.';con-: ma1ly:be:leakingi into the vacuum system. templated to-use as the probe medium: any 5111 One form ofz'detecting means-which can be emable a gas which is nontoxicye'nonexplosive.-tand p yed-ior'z the purposesin vieW-is-th'e mass'spec--'--- free from noxious 1 sme11;:su'ch afor exampleaasi trometen-yinasmuchsas.it can be arranged to -re- 15 carbon'dioxide; 'substances 'eminently suited .ior. spondonly-'to1a preselected gas, sueh-forexam 1e-- absorbing'-- carbon dioxidezby chemicaliaction as helium zofi massi i or the methyl radicah of therewithare the hydroxides: of. the alkaline methane. :However; a serious disadvantage-of earth metals such as barium hydroxide and calthe massspectrometer is itsrelatively high" cost;-"- cium 1'1ydroxi'de;

In another '.;arrangement-which involves *the In practising-the inventionsitistpreferredto1. use asthe probe:medium. of a-g'as which is con-'- employ' e1ectroeresponsive gaugesofithe thermal ":1 densable in-.1iquid.,air, the detecting means =inconductivity oriionization types; .=;rAdvantageous-' eludes two Pirani:gauges-which are identicalex w ly, the gauges.may' -be housedain;separate"com-w:1 ceptkthatione has aniassociatedliquidair trap; artments of a, common housing whictnprefera-bly the gauges are differentially. -connected i11 an includes an additiona'lecompautment\for accorm" electric bridge network sothat the bridge is normodation' of' the ehemi'eahireagent.' :Access @ofs mally-in,ibalanceznotwithstanding leakage ofeir 1 gases' withi-rr the vacuumisystem to'rthe compart' into and fluctuations ofv pressure in, the vacuum ment containingsthe chemical reagent and from systemrwhereas;whenthe probe medium is apthis compartment toithe eompartmentsior one plied-:to:the exterior-of the vacuum system; any 0 of. the gaugeszasalsothe direcltraecess to the leakagerof said m'ediumiiinto the vacuum system otheri 'gaugepis p w yfifio u fiOW:

becomes :cond'ensed in'the liquid-air trapand constructors? suitably rad-justed -ato-give;the besto produces a: fall of pressure at the associated performances. Pirani-gauge'swhile the pressure- 'at the other- An obiectrof this invention-Lie toeprovide i-mu exremains?sensiblytunaltered; with=-the re- 5 proved, relatively inexpensive apparatus for desult vthataithe bridgebecomes 'unbalanced thus tectingleaks-in a yaeuumsystemes-other.objects indicating :the existence and extent 0f: leakinto andmadvantagesewi-ll yapp aireas. the dBSCIiIYBiOI-M-V the vacuum-system; :Among the disadvantages roceeds. :mFeatures-of-the inventioniwhichiare of this arrangement are those invo1vedin the be1ieved .to ..be hovel-Hand pate tableere pointed use ofgliquid air; namely the relatively higheo'st out innolaimsmwhichiiormiaipart ofthi's spec1fi. and the diflicultyv of procuring liquid air, andalso cation.

its tendency .torproduce driftsfiin the sensitive=*"-' Forea.bettereunderstandingi of. the invention 1. elements which may pers st or Several u s a referenceois madein'the, following description to ter commencement aof the l'eakage d'etection." the accompanying,drawing in which the single" Moreover;whereas'rthe 'conden'sable; gas most 5 figure is a schematic diagram; partiy in seetion, satisfactorypfrom the point of view ofileakage of a preferredembodimeht'oi the invention.

detection :is butane; this gas when used in ap- Referring to thed' wi sf l p p f W2111S preciableiquantity hasanobjectionable:smell and defining "a first passageway-2v 'A housing D v alsomay-giverise torexplosive atmospheres5"- creamy-comprising'a-coppex bloele 3-ha-s walis- According. tothe present-invention, in animdefiningo;secondpassageway ewhiehpasses provedzarrangementifor the detectionof 'leakagethrough-a 'oentrai'portionof the biock'asfi l-l-usinto1a.;vacuumzrsystem,' ztwo electro-respons-ivetrated, and two recesses5 and 6;."on'e oil-either vacuum;:gauges are arrangedsone to havedirecu side'p'f. passageway 4:"7Gne end 'ofp ot y'f 'f communicationiwitha the evacuated space of the may-be-closedby a-tubuiar-m-ember-i Which-is vacuumzsystemiand the other-for communication threaded= toreceiveaeremovable eap 8"..= An airtight seal is provided by gasket 9. The other end of passageway 4 is connected to one end of pipe I as shown. The other end of pipe i may be provided with a suitable connector H! for con necting passageway 2 with the vacuum system.

Between passageway 2 and passageway lthere is a flow-constricting plug I I having a small axial bore l2 connecting the two passageways. Block 3 has a small transverse bore 13 forming a flowconstricting duct between recess 5 and a portion of passageway 4 remote from plug H. Block 3 has another small transverse bore Hi forming a flow-constricting duct connecting recess 6 to passageway 2.

Vacuum gauges are located in recesses 5 and 6. Preferably Pirani gauges are employed. One of the Pirani gauges comprises resistance wires l5 and I5 connected in series between leads H and I8 and supported by a supporting member I9. Leads H and it pass through a glass seal 29 which closes recess 5. Within recess 53 there is a similar Pirani gauge comprising resistance wires 2| and 22, support 23, leads 2:: and 25, and glass seal 26.

When an electric current is passed through the resistance element of a Pirani gauge, the rate of heat transfer from the resistance element, and hence the temperature of the resistance element, varies as a function of gas pressure within the recess containing the gauge. These temperature changes of the resistance element alter its electrical resistance, which may be measured by a suitable electrical circuit, A more complete discussion of Pirani gauges may be found on pages 318-330 of the book Scientific Foundations of Vacuum Technique, by Saul Dushman, published by John Wiley and Sons, Inc., New York, 1949.

Preferably the two Pirani gauges are connected in adjacent branches of an electrical bridge circuit comprising galvonometer 2i, tapped resistor 23, and voltage source 29 connected as shown. The bridge circuit may be balanced by adjusting tap 30 of resistor 28.

Passageway d is loosely packed with an absorbing material 3! which chemically absorbs the probe medium but which does not absorb air. When carbon dioxide gas is used as the probe medium, the absorbing material may be a hydroxide of an alkaline earth metal, as hereinbefore explained. Preferably, the absorbing material is in the form of sintered pellets held together by a suitable binder to give a less friable absorbent and more consistent gas flow conditions.

The flow-constricting ducts I2, It, and M are designed to give sensibly equal gas flow time constants to the two gauge elements so that both long-term and short-term changes of the mean background pressure within the vacuum system are compensated.

In use, the apparatus is connected by means of connector Ill to a vacuum system to be tested for leaks. Preferably, the electrical bridge circuit is initially balanced by adjustment of tap 30 for zero indication of galvanometer 21. The probe medium, which may be carbon dioxide gas, is then sprayed upon portions of the vacuum system to be tested for leaks. If there is a leak in such portions, some of the probe medium enters the vacuum system.

Normally, any air leak into the vacuum system which passes through inlet pipe I is subdivided into two components, one component passing directly into recess 6 and the other component passing through absorbing material 3| into recess 5. Since air is unaffected by the absorbing material, it has the same eiTect upon both Pirani gauges, and the electrical bridge remains in balance. However, when any of the probe medium enters the vacuum system and passes into inlet pipe I, the probe gas enters recess 6 but does not enter recess 5, since the probe medium reacts chemically with absorbing material 3!. Since the probe gas does not enter recess 5, but air within the recess continues to diffuse out through flow-constricting duct It, the pressure within recess 5 decreases, while the pressure within recess 8 remains constant or may rise slightly. Thus, presence of the probe medium creates a pressure difierence between the two recesses 5 and 6. This pressure difference acts upon the two Pirani gauges to unbalance the electrical bridge circuit, whereby presence of the probe medium in the vacuum system is indicated by galvanometer 21.

Having described the principle of this invention and the best mode in which we have contemplated applying that principle, we wish it to be understood that the example described is illustrative only, and that other means may be employed Without departing from the true scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for detecting leaks which admit a gaseous probe medium into a vacuum system, comprising walls defining first and second passageways, means connecting said first and second passageways, means for connecting said first passageway to the vacuum system, gas-absorbing material within said second passageway, said material being of a kind which absorbs the probe medium but does not absorb air, first and second vacuum gauges, means connecting said first vacuum gauge to said first passageway, and means connecting said second vacuum gauge to the end of said second passageway not connected to said first passageway.

2. Apparatus for detecting leaks which admit a probe medium of carbon dioxide into a vacuum system, comprising walls defining first and second passageways, flow-constricting means connecting said first and second passageways, means for connecting said first passageway to the vacuum system, absorbing material within said second passageway, said absorbing material being of a kind which absorbs carbon dioxide gas but does not absorb air, first and second vacuum gauges, flow-constricting means connecting said first vacuum gauge to said first passageway, and flow-constricting means connecting said second vacuum gauge to the end of said second passageway remote from said first passageway whereby a medium must travel through said absorbing material before reaching said second vacuum gage.

3. Apparatus for detecting leaks which admit a probe medium of carbon dioxide into a vacuum system, comprising walls defining first and second passageways, flow-constricting means connecting said first and second passageways, means for connecting said first passageway to the vacuum system, absorbing material within said second passageway, said absorbing material being a hydroxide of an alkaline earth metal, first and second vacuum gauges, flow-constricting means connecting said first vacuum gauge to said first passageway, and flow-constricting means connecting said second vacuum gauge to the end of said second passageway remote from said first passageway whereby a medium must travel through said absorbing material before reaching said second vacuum gage.

4. Leak-detecting apparatus for detecting the presence of a probe medium in a vacuum system, comprising walls defining first and second passageways, flow-constricting means connecting said first and second passageways, means for connecting said first passageway to the vacuum system, absorbing material within said second passageway, said absorbing material being of a kind which chemically absorbs the probe medium but does not absorb air, first and second Pirani gauges, flow-constricting means connecting said first Pirani gauge to said first passageway, and fiowconstricting means connecting said second Pirani gauge to the end of said second passageway remote from said first passageway whereby a medium must travel through said absorbing material before reaching said second Pirani gage.

5. Apparatus for detecting leaks which admit a probe medium into a vacuum system, comprising walls defining first and second passageways, flow-constricting means connecting said first and second passageways, means for connecting said first passageway to the vacuum system, absorbing material within said second passageway, said material being of a kind which absorbs the probe medium but does not absorb air, an electrical bridge circuit, first and second Pirani gauges differentially connected in said bridge circuit, fiowconstricting means connecting said first Pirani gauge to said first passageway, and flow-constricting means connecting said second Pirani gauge to the end of said second passageway remote from said first passageway whereby a medium must travel through said absorbing material before reaching said second vacuum gage.

6. Apparatus for detecting leaks which admit a probe medium into a vacuum system comprising an inlet pipe having walls defining a first passageway, a block having walls defining two recesses and a second passageway extending through the block, an air-tight seal for closing one end of said second passageway, the other end of said second passageway being connected to one end of said inlet pipe, a connector at the other end of said inlet pipe for connecting said first passageway to the vacuum system, a flowconstricting plug having a small bore therethrough between said first and second passageways, said block having a small bore forming a flow-constricting duct between one of said recesses and a portion of said second passageway remote from said plug, said block having another small bore forming a flow-constricting duct between the other of said recesses and said first passageway, vacuum gauges respectively located in said two recesses, and absorbing material within said second passageway, said material being of a kind which absorbs the probe medium but does not absorb air.

7. In a leak detecting apparatus for detecting the presence of a probe medium in a vacuum system, the combination comprising inlet means defining a first passageway, a housing having wall sections defining a second passageway communicating with said first passageway and two recesses adjacent the second passageway, said wall sections having bores therein communicating between one of said recesses and one end of said second passageway and communicating between the other of said recesses and the remaining end of said second passageway with one of said bores having direct access to the first passageway defined by said inlet means, absorbing material disposed within said second passageway intermediate said bores, said absorbing material being of a kind which chemically absorbs the probe medium but does not absorb air, a first vacuum gage operatively connected to one of said recesses, and a second vacuum gage operatively connected to the remaining one of said recesses.

8. The combination set forth in claim '7 wherein said vacuum gages constitute Pirani gages having the hot wire elements thereof disposed within the respective recesses and electrically connected in a measuring circuit arrangement.

JACK BLEARS.

JOHN HENRY LECK.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,644,951 Rodhe Oct. 11, 1927 2,263,335 Heinz Nov. 18, 1941 2,506,535 Zaikowsky May 2, 1950 2,507,321 Sherwood May 9, 1950 FOREIGN PATENTS Number Country Date 824,090 France Feb. 1, 1938 

